1. Field of the Invention
This invention relates to an ophthalmic examination apparatus, and more particularly to an electronic type ophthalmic examination apparatus which uses a laser beam for two-dimensional scanning of the eye fundus, collects the light reflected back from the eye fundus and subjects the light to photoelectric conversion to obtain information about the eye fundus.
2. Description of the Prior Art
Conventionally, in order to examine the eye fundus there are in wide use the method whereby the physician examines the patient's eye directly by means of an opthalmoscope, and the method whereby a special fundus camera is used to take photographs of the eye fundus. Also, with the advance in recent years of electronic technology, use is also being made of optoelectronic transducers such as imaging tubes and the like in place of the photographic film of the conventional fundus camera, eye fundus information is read out directly in the form of electric signals which are processed and stored in a memory or displayed on a monitor television or the like.
Of these conventional electronic examination apparatuses, one that employs laser scanning and which has developed by the Retina Foundation of the U.S. (see Applied Optics, vol. 19 (1980) page 2991) has attracted attention for the many features it possesses. Specifically, by replacing the light source conventionally used in the flying spot scanning type video image input system by a laser beam for eye fundus applications, restricting the incident light beam to a small zone in the center of the pupil and receiving, photoelectrically converting and amplifying the light reflected by the eye fundus from a larger area around the periphery of the pupil, it becomes possible to display on a monitor television a real-time video image of the eye fundus with a low brightness and a high S/N ratio. In addition, it becomes possible to decrease greatly the amount of fluorescent agent that is administered when fluorescent image photography of the eye fundus is to be performed. Also, by modulating the scanning laser beam it becomes possible to examine retina function in the course of observing the eye fundus image, and by utilizing the advantages of the laser beam's depth of focus, the elimination of corneal reflection due to polarization and the monochromatic nature of the light, it becomes possible to provide an excellent diagnostic apparatus.
The drawback with this type of apparatus is that the system for controlling the laser beam deflection is difficult. In the reference material cited in the above, two mechanical laser beam deflection systems are employed which are operated at scanning frequencies of 7.8 kHz for the horizontal scanning and 60 Hz for the vertical scanning. But, in order to obtain high-definition video images it is necessary to use higher laser beam scanning frequencies. In the case of laser beam deflection corresponding to standard NTSC raster scanning, the horizontal scanning frequency is 15.75 KHz, which is basically impossible to realize with a mechanical type of optical deflector from the standpoint of service life and durability.
The practical application was then tried of an idea that was announced which involved the use for the horizontal deflector of a non-mechanical acousto-optical device having no moving parts. If, however, the apparatus is to be used to obtain color information, because the acousto-optical device utilizes diffraction, the angle of deflection inherently differs in accordance with the color, and compensating for this requires an extremely complex optical system. In addition, each such type of optical system is effective only with respect to a set laser beam wavelength. Thus, as the eye fundus is comprised of a plurality of layers each of which has reflection characteristics that differ from those of the other layers with respect to a set wavelength, in order to accurately diagnose morbid portions it is necessary to observe the eye fundus with light of each of the necessary wavelengths. However, in the case where an acousto-optical device was employed for the laser beam deflection, it was impossible to change to a laser beam having any desired wavelength among an arbitrary plurality of wavelengths.
It is therefore an object of this invention to solve the aforementioned problems by providing an ophthalmic examination apparatus that enables the realization of a laser beam scanning system that makes it possible to realize high-frequency laser beam scanning, can be freely adapted to any arbitrary wavelength, allows color information to be obtained and has outstanding reliability and operability.